No hextech chest with S rank

Did you already get one for that champion?

Also, you do not always get a chest with an S.

You can check if you got the chest by looking at your profile, and then champions. If they have a chest boarder then you got one recently.

Also you can check the profile and see if you have any chests available on the main profile page.

Have you already received a Hextech Chest with Morgana earlier this season?

You only get one per season for each champion.

If you go to your champions page you might see something like this

https://support.riotgames.com/hc/article_attachments/115008355987/Champion-Mastery-Chest-Earned.jpg

Notice how Poppy has a border around her icon. That means this player has already received a Hextech Chest with Poppy for this season.

It also says in the tooltip.

The other champions do not have this border because their available to receive a Hextech Chest.

Do you own Morgana, or do you have a chest available?

You most likely didn't have a chest available. You do only get 4 a month.